Salut,
Dans tous les cas, on additionne la masse molaire de chaque atome de la molécule/l'ion (donnée dans la classification périodique) :
a) M(H2O) = 2×M(H) + M(O)
= 2×1,0 + 16,0
= 18,0 g/mol
M(CO2) = M(C) + 2×M(O)
= 12,0 + 2×16,0
= 44,0 g/mol
M(C2H6O) = 2×M(C) + 6×M(H) + M(O)
= 2×12,0 + 6×1,0 + 16,0
= 46,0 g/mol
b) M(Na+) = M(Na) =23,0 g/mol
M(CH-) cet ion me dit rien, jsp si c'est une erreur mais si tu suis la technique ce serait M(C) + M(H)
= 12,0 + 1,0
= 13,0 g/mol
M(H3O+) = 3×M(H) + M(O)
= 3×1,0 + 16,0
= 19,0 g/mol
M(HO-) = M(H) + M(O)
= 1,0 + 16,0
= 17,0 g/mol.
Voilà, bonne journée :)
